Capacitive physical quantity sensors, which have at least one movable electrode and at least one pair of fixed electrodes placed to be opposite thereto and sense a physical quantity of a target based on a capacitance between the movable electrode and the fixed electrodes, have been applied for vehicle's acceleration sensors.
As an example of such capacitive physical quantity sensors, the structure of a capacitive physical quantity sensor, which is disclosed in U.S. Pat. No. 6,257,061 corresponding to Japanese Patent Publication No. 2000-81449, is illustrated in FIG. 10.
The capacitive physical quantity sensor illustrated in FIG. 10 is provided with a capacitive physical quantity sensor element 110 composed of a movable electrode 111 and fixed electrodes 112a and 112b. The paired fixed electrodes 112a and 112b are placed to be opposite to the movable electrode 111. The capacitive physical quantity sensor is also provided with a C-V (Capacitance-to-Voltage) converter 120 for converting changes in capacitance sensed by the sensor element 110 into a corresponding voltage signal, thereby outputting the voltage signal.
Moreover, the capacitive quantity sensor is provided with a sample and hold circuit 130 configured to sample the value of the voltage signal at predetermined intervals of time and to hold each sampled value. Furthermore, the capacitive quantity sensor is provided with an amplifier 140 for amplifying a discrete voltage signal output from the sample and hold circuit 130, and a low pass filter (LPF) 170 for eliminating higher-order components contained in the amplified voltage signal therefrom.
A similar structure of a capacitive physical quantity sensor, which is substantially identical with that of the capacitive physical quantity sensor illustrated in FIG. 10, is disclosed in U.S. Pat. No. 6,668,614 corresponding to Japanese Patent Publication No. 2003-121457.
In addition, in U.S. Pat. No. 5,633,594 corresponding to Japanese Patent Publication No. H08-145717, the structure for eliminating the influence of an amplifier or a switching transistor is disclosed.
In order to amplify the minute voltage signal output from the C-V converter 20 of the capacitive physical quantity sensor illustrated in FIG. 10, the gain of the amplifier 40 can be set within the range from dozens of decibels to several hundred thereof. The gain characteristic of the amplifier 40 varies depending on change in ambient temperature.
For correcting the change of the gain characteristic of the amplifier 40 depending on change in ambient temperature, the gain characteristic of the amplifier 40 might be previously measured within a usable ambient temperature range so that the measured gain characteristic may have been stored in a memory unit, such as a ROM (Read Only Memory).
In addition, because of age deterioration of operating characteristics of the C-V converter 20, the sample and hold circuit 30, and the amplifier 40, measurement accuracy of the capacitive physical quantity sensor may deteriorate over time.